"Steam has been a service exclusively for PC users since we
launched it in 2004. For the first time we and our partners are
bringing it to another platform," said the Valve team on
the company blog. "It's been a ton of work, but the Mac is
great for the same reason the PC is great - they are both open
systems that let gamers and game developers be as close as possible."

There are currently 57 games available to purchase and download for gamers using the Steam client for OS X.

Gamers will be able to play purchase a game once and play it on
both Windows and OS X platforms. In addition, thanks to Steam
Play, a game can be started on a PC and finished on a Mac or vice
versa during any part of the single-player game experience.

In celebration of the Steam client being made available for OS X,
Valve is making
Portal available for free until May 24. "Like we keep
saying, Portal is free. Free on the Mac," Valve explains.
"Free on the PC. But only until May 24th. So you only have a few
days to decide if your free copy of Portal is worth the price
we're currently charging."